This invention relates to a laminated magnetic rotor for a motor and a method of fabricating the rotor laminations, and more specifically to the construction of composite disk laminations for use in reluctance, homopolar or interior permanent magnet rotors.
Reluctance, homopolar and interior permanent magnet machines share the property of requiring magnetic saliency. In the reluctance and homopolar machines this always implies geometric saliency. the result is high windage losses, especially since the machines are favored for high speed. To achieve high strength, solid rotors are often employed; however, this leads to large pole face losses due to slot ripple and, for the switched reluctance motor, flux exclusion and intolerable rotor core losses, resulting in loss of efficiency and local heating. The salient poles also incur considerable aerodynamic or windage losses. Thus, it has become desirable in reluctance, homopolar and interior permanent magnetic machines to construct the rotor of a stack of composite disk laminations.
An example of a prior art homopolar machine is seen in high speed flywheel energy storage systems, such as are useful in generating mobile electrical power for vehicular transportation. These energy storage systems have stringent limitations on volume and weight of the power generating system which necessitate use of small, lightweight alternators with high power output capabilities. This in turn requires that the alternator operate at high speeds. Conventional wound rotor alternators, however have been limited in operating speed since, at high angular velocities, typically above about 26,000 rpm, mechanical stresses in the rotor windings become excessive. Solid rotor machines, therefore, have heretofore been necessary since, at the high angular velocity of operation, only a solid rotor could withstand the high mechanical stresses thereby incurred. Typical of such solid rotor machines has been the homopolar inductor alternator. In such machines, the rotor has carried no windings and has basically been comprised only of magnetic material, thus facilitating rotor operating speeds in excess of 90,000 rpm for small machines.
In FIG. 1, a conventional prior art rotor 10 for an eight-pole homopolar inductor alternator is illustrated. This rotor, comprised of solid magnetic steel, has been employable with a conventional wound stator (not shown) of the type described by E. Richter in the Conference Proceedings of the 1971 Intersociety Energy Conversion Engineering Conference, Boston, Mass., pages 132-139, Aug. 3-5, 1971. The rotor is formed with two general regions 11 and 12, each region including half the total number of rotor poles. The salient poles in region 11 are circumferentially offset from the salient poles in region 12 by an angle .theta. which is defined as 360 degrees/N, N being the total number of poles on the rotor. For an eight-pole rotor as shown in FIG. 1, angle .theta. equals 45 degrees.
The only magnetic material required for proper rotor operation is that which provides flux paths between corresponding pole faces, such as faces 13 and 14 of the rotor shown in FIG. 1. A typical magnetic flux path in this material as illustrated in FIG. 2A as it would appear if the rotor, shown in FIG. 1, is viewed longitudinally from the left end. Any more magnetic material other than similar segments typified by that shown in FIG. 2A is extraneous to the magnetic circuit for the flux path furnished by the material of FIG. 2A, and can only increase the leakage flux. FIG. 2B illustrates rotor magnetic flux paths when the rotor is viewed from the side.
During operation of the homopolar alternator containing rotor 10, magnetic flux which enters a pole face in one region exists the opposite complementary pole face in the other region. Specifically, magnetic flux entering pole face 13 may exit at pole face 14. Minor variations in the flux passing through pole faces 13 and 14 due to relative motion of the stator induce eddy current flow in the faces, causing electrical loss and attendant heating of the pole faces. Moreover, since the entire rotor is magnetic, there exists a large amount of magnetic flux that does not pass through the air gaps located between the stator and the pole faces and on the rotor. This flux tends to saturate the rotor arm without contributing to the alternator output. Efficiency of the machine is thereby reduced.
One method of alleviating the aforementioned problems arising due to eddy currents in the rotor of FIG. 1 involves use of planar laminations in the manner illustrated in FIG. 3. This construction allows much of the rotor intermediate the regions containing the poles to be fabricated of non-magnetic material, so that leakage flux can be reduced. Specifically, a solid ring 25 of magnetic material abuts, and is situated between, first and second stacks 26 and 27, respectively, of planar laminations of magnetic steel in order to form a low reluctance path between radially-consecutive poles formed by the laminations 23 and 24, respectively and separated axially. A non-magnetic spacer 28 encircles ring 25 and helps maintain lamination stacks 26 and 27 packed tightly together and oriented normal to longitudinal axis 30 of rotor shaft 31. The stacks of laminations are urged toward each other axially by non-magnetic end clamps 35 and 33 in a manner well known in the art. This configuration, by employing non-magnetic spacers to help keep the radially-outer portions of lamination stacks 26 and 27 uniformly spaced apart from each other, avoids the necessity of having to add iron around the magnetic ring 25, thereby holding the amount of leakage flux to a low value.
While welding of the laminations to a solid part is not critical in the rotor shown in FIG. 3, another form of magnetic machine which requires laminations has properties in which such welding is critical. In the latter form of machine, such as typified by that shown and described in Miller et al. U.S. Pat. No. 4,464,596, issued Aug. 7, 1984 and assigned to the instant assignee, solid, axially-extending non-magnetic inserts are welded to laminated pole pieces. A limitation on operating such rotor at very high speed has been the insufficient strength of the bond between dissimilar materials in the region of the inductor rotor which is highly stressed by centrifugal force. This bond typically involves welding or brazing of the stacks of laminations to solid members. Another factor that has limited use of rotors requiring bonds between dissimilar materials has been the difficulty in achieving adequate penetration of the weld between such materials, and especially the need for extensive skilled manual direction and setup to make the weld. For example, homopolar rotors comprised of axially-stacked laminations of magnetic material with solid, non-magnetic interpole sections welded axially to the stacked laminations have been fabricated, but such rotors have not been commercialized because of their high cost of construction resulting from the inability to automate their fabrication. It would be highly desirable to overcome these limitations of speed and high cost in the rotor of homopolar machines.
Turning now to a brief discussion of reluctance magnetic machines, switched reluctance motors conventionally have poles or teeth on both the stator and the rotor (i.e., they are doubly salient). There are phase windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator poles is connected in series to form one phase of the switched reluctance motor. Torque is produced by switching current on in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase at the commutation point before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction will produce a negative or braking torque.
The switched reluctance magnetic machine rotor has many of the same problems as the homopolar rotor. Therefore, it would be highly desirable to overcome the limitations of prior art reluctance rotors in speed and in the high cost of fabricating such rotors.
With reference to interior permanent magnet machines, rotors of this type of machine have similar problems to overcome. For example, to overcome eddy currents in interior permanent magnet machines, a rotor has been comprised of a stack of disk laminations. An example of such a prior art rotor is shown in Jones U.S. Pat. No. 4,486,679, issued Dec. 4, 1984 and assigned to the instant assignee. The rotor of the Jones patent is comprised of a stack of disk laminations, each of the laminations having a plurality of pole piece sections connected to one another by circumferential bridges situated at the disk periphery, with the pole piece sections and interior core portion defining the radial thickness of magnet slots. Radial ligaments situated on each disk connect the interior portion of the disk lamination to the circumferential bridges. In fabricating the rotor, the disk laminations are stacked so that the magnet slots extend through the stack in an axial direction. The magnets are placed in the magnet slots and the bridges are pressed inwardly, plastically deforming the bridges and ligaments, creating a predetermined hoop stress which holds the magnets in their respective slots when the pressing force is removed. However, the strength, and hence maximum rotor speed, is determined by the stress in the bridges. These bridges must be kept as thin as possible in order to minimize the amount of magnetic flux required to saturate them to assure proper operation of the motor. Since the bridges are of the same magnetic material as the rest of the rotor, their tensile strength is low, limiting the maximum operating speed of the rotor. Therefore, it would be highly desirable to increase the strength of the bridges and reduce their magnetic effect and thus increase both the speed capability of the rotor and the specific torque of the motor.